1. Field of the Invention
The present invention relates to a medical X-ray apparatus and an irradiation tube. In particular, it relates to a dental X-ray apparatus such as oral radiographic X-ray apparatus as well as to an irradiation tube for use in the dental X-ray apparatus. In addition, the invention relates to a light source control circuit for lighting a light source for a specific period of time which can be incorporated in the medical X-ray apparatus and the irradiation tube.
The invention further relates to a medical position indicating apparatus for indicating an irradiating position of invisible electromagnetic waves such as X-ray, infrared ray and ultraviolet ray used in the medical care field.
2. Description of the Related Art
The dental X-ray apparatus for oral radiography is the only radiographic apparatus legally permitted to emit X-ray to a radiographic film being inserted into the oral cavity in a region of a subject larger than the size thereof. For an ordinary film size of 30.times.40 mm (film diagonal size 50 mm), in various standards irradiation exceeding the film diagonal size at the end portion of an irradiation tube is permitted in the oral radiography. However, such irradiation is not allowed in other radiographic applications.
This is because, in the dental X-ray apparatus for oral radiography, unlike other radiographic apparatuses, the apparatus and film are not always used at a specific dimension, but the angle and distance vary with the desired radiographic position, and moreover, since the film is put into the mouth of a patient, the film position cannot be directly observed by a radiologist. Also, a radiation cone of X-ray, that is, a radiation field does not fit within the film, and it often causes a radiographic error referred to as "cone-cut." When a cone-cut occurs, it is necessary to take an X-ray again, which leads to increased exposure dose of the patient. To reduce this risk, it is advisable to emit an X-ray in a wider area exceeding the film size. Thus, in the dental X-ray apparatus for oral radiography, even if the exposure area is determined to be slightly greater than the film size, the total exposure dose of the patient can be reduced as compared with multiple exposures of X-ray.
However, even in such dental X-ray apparatus permitted to emit X-ray in a region exceeding the film size, it is desired that the number of exposures be decreased, and the exposure dose and exposure area also be reduced, from the viewpoint of reduction of the exposure dose of the patient. It is necessary, for this purpose, to improve the positioning performance and positioning reproducibility of the X-ray apparatus. As used herein, "the positioning reproducibility" refers not only to the positioning reproducibility of X-ray irradiation field, but also to the positioning reproducibility of an irradiation angle. Such reproducibility is important in comparing observations before and after the operation or the treatment of a patient. In the prior art, a dental X-ray apparatus includes the indications of graduations so that the irradiation angle of the apparatus can be determined by a radiologist. This, however, does not sufficiently improve the reproducibility. To enhance the reproducibility, attempts have been made to optically determine a position with visible light.
Typical prior art is disclosed in the Japanese Unexamined Utility Model Publication (KOKAI) No. JP-U 55-51280 (1980). In this prior art, a visible optical system for displaying a visible spot light in the X-ray irradiation position of a subject is constructed so as to move freely in and out between a position coaxial with the X-ray irradiation axis of a irradiation tube, and a retracted position outside the irradiation region.
However, since the visible optical system is moved mechanically, the position of the visible spot light may vary every time because of dimensional errors or assembly errors of the moving and stopping mechanism. The reproducibility is thus expected to be poor. Also, in observing the visible spot light, it is hard to pin-point the position and direction of the X-ray irradiation field.
Further, after positioning with spot light, it is necessary to put aside the visible optical system mechanically to the retracted position, and therefore, the positioning once set may be deviated by the retracting action.
In this prior art, the position of focusing the spot light, that is, the position where the diameter of the spot light is the minimum, is a certain point in the optical axis direction, and the diameter of the spot light greater in the front and rear of the focus point than that at the focus point. When positioned by emitting spot light to the subject at a relatively shallow angle, that is, when emitting spot light obliquely, not from the front of the subject, the emitted spot light may be elliptical, or the spot light shape may be distorted by the body of the subject, such as a facial shape or skin asperities. It will be then difficult to accurately determine an optical axis, that is, the axis of X-ray.
Another prior art is Japanese Unexamined Utility Model Publication (KOKAI) No. JP-U 55-166213 (1980). This prior art reference discloses an optical conductor body in a tubular shape which approximates a radiation cone of X-ray. It is designed to emit light so as to draw a contour of the X-ray irradiation field with visible light ray. However, since the contour shape of sighting beam is a ring form, it is difficult to clearly judge the center of the X-ray irradiation field.
A further prior art reference is Japanese Unexamined Utility Model Publication (KOKAI) No. JP-U 58-1405 (1983), in which spot light is emitted to a certain point on the axis of X-ray from three positions provided at intervals in the circumferential direction of the irradiation tube. In this prior art, a plurality of irradiation spot lights appear in the front and the back of the focus point on the axis of the X-ray. Therefore, in this prior art, it will also be difficult to judge the center of irradiation field.
In a conventional X-ray irradiation tube, to illuminate a light source for generating a sighting beam, a power switch attached to the irradiation tube or X-ray apparatus must be manipulated. Types of power switch include: 1) an automatic reset switch which closes contact when the switch is pressed and which opens contact when released; and 2) a toggle switch which keeps contact closed or open by the latch mechanism of lever and the like. When the automatic reset type switch (1) is used as a power switch, the irradiation tube must be positioned while pressing the switch, and the operation is thus complicated and the controllability is poor. In the toggle switch (2), once the switch is on, the positioning operation can be done without any further action. However, when the switch is turned off after positioning, an extra force may prevail and the position may be deviated. Power may be wasted if the switch is forgotten to be turned off.
To have the X-ray irradiation tube to be detachable from any desired type of X-ray apparatus, a battery or power supply unit must be built in the irradiation tube itself. In the X-ray apparatus, a light source starting circuit of power-saving type is indispensable to increasing the number of exposures of the sighting beam. In this case, if output voltage is lowered gradually due to discharge characteristics of the battery, the light source exposure quantity fluctuates and the illumination of the sighting beam varies. Thus, the controllability is impaired. To replace for an X-ray irradiation tube without a boresight mechanism, a small and lightweight X-ray irradiation tube is required.